The dynamic conformation of the peptide hormones oxytocin and (8-arginyl) vasopressin are being investigated by nuclear magnetic resonance studies of specifically designed and synthesized isotopic isomers in aqueous solution. These isomers contain substitutions of deuterium for protons in order to remove overlapping resonance patterns of similar chemical shifts, to simplify spin systems in which the participating nuclei are either so numerous or so close in chemical shift that analysis is rendered impossible, or to stereospecifically identify particular protons. Additionally, carbon-13 and nitrogen-15 substitutions are used so that vicinal heteronuclear coupling constants can be observed in the proton spectrum. The objective of these studies is to investigate the structure - function relationships of these peptide hormones by conformational studies, and to develop methods for investigating the role of dynamic conformations in peptide hormones, in the action of enzymes, and in the interactions of peptides and proteins with nucleic acids and lipids.